This invention is based on and claims priority of Japanese patent application 2001-331091, filed on Oct. 29, 2001, the whole contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical signal processing device for carrying out signal processing without converting an optical signal into an electrical signal, an optical demultiplexer, a wavelength converting device, an optical signal processing method, and a wavelength converting method.
2. Description of the Related Art
Recently, a wavelength division multiplexing (WDM) optical communication system has been developed as a broadband optical communication system. Other optical communication systems, such as optical time division multiplexing (OTDM) and time wavelength division multiplexing (TWDM), have also been proposed and studied aiming at broader band optical communication.
In the WDM optical communication system, a plurality of wavelengths of wavelength multiplexed optical signals are assigned to communication channels in a one-to-one relation. For multiplexing a plurality of optical signals, each of original optical signals to be multiplexed must be converted to have the wavelength of a communication channel to which the optical signal is assigned. Hitherto, such wavelength conversion has been performed by first converting the optical into an electrical signal and then converting the electrical signal into an optical signal of a desired wavelength.
In the OTDM optical communication system, signal density is increased by employing optical pulses having the same wavelength and having a very narrow pulse width. Communication rate of a time-division multiplexed optical signal is, for example, 160 Gbits/s or higher.
Response speed of an electrical signal is limited by a moving time of carriers in a semiconductor device and hence lower than the response speed of an optical signal. At present, the speed limit of an electrical signal is thought to be about 40 Gbits/s. To process an OTDM signal having speed higher than that limit, an optical signal must be divided through high-speed optical signal processing and demultiplexed to a bit rate, at which electrical processing is feasible.
The TWDM optical communication system as a combination of the OTDM optical communication system and the WDM optical communication system is able to realize broader band optical communication.
In view of the above-mentioned background, an optical device (optical demultiplexer) has recently been studied which is able to demultiplex an optical signal, as it is, without converting the optical signal into an electrical signal. Hitherto, optical demultiplexers of, e.g., non-linear optical loop mirror (NOLM) type, Mach-Zehnder type and polarization separating type, have been proposed.
FIG. 9A is a schematic view of a NOLM type optical demultiplexer. An optical signal sig1 reaches a branch point 102 of an optical fiber loop 101 via an input side optical fiber 100. At the branch point 102, the optical signal sig1 is branched into an optical signal sig2 propagating in the loop 101 counterclockwise and an optical signal sig3 propagating in the loop 101 clockwise. The optical signal sig1 is a signal having four time-division multiplexed channels, i.e., channels #1 to #4.
A non-linear waveguide 103 is inserted in the optical loop 101 at a position asymmetrical to the branch point 102. The optical signal sig2 propagating counterclockwise reaches the non-linear waveguide 103 at timing earlier than the optical signal sig3 propagating clockwise. A control light pulse con is inputted to the non-linear waveguide 103 immediately after the channel #2 of the optical signal sig2 has passed the non-linear waveguide 103. The refractive index of the non-linear waveguide 103 is changed upon the inputting of the control light pulse con, whereby the phase of a pulse light in each channel #3 and #4 of the optical signal sig2 is shifted xcfx80. In FIG. 9A, a pulse having phase shifted xcfx80 is represented by hatching.
Because the optical signal sig3 reaches the non-linear waveguide 103 at timing delayed from the optical signal sig2, only the channel #1 of the optical signal sig3 has passed the non-linear waveguide 103 at the time when the control light pulse con is inputted to the non-linear waveguide 103. Therefore, the phase of a pulse light in each of the channels #2 to #4 of the optical signal sig3 is shifted xcfx80.
When the optical signals sig2 and sig3 return to the branch point 102, the pulses in those ones #1, #3 and #4 of the channels of both the signals, which are in phase, propagate in the input side optical fiber 100, and the pulse in the out-of-phase channel #2 propagates in an output side optical fiber 105. Thus, only the signal of one channel can be separated from the time division multiplexed signal sig1.
In the NOLM type optical demultiplexer, the time required for the optical signal to pass the optical loop 101 limits the signal speed achievable in signal processing. Also, the use of an optical fiber loop raises a difficulty in reducing the device size.
FIG. 9B is a schematic view of a Mach-Zehnder type optical demultiplexer. Non-linear waveguides 121 and 122 are inserted respectively in two arms of a Mach-Zehnder interferometer 120. An optical signal sig10 is branched into two optical signals sig11 and sig12, which are introduced to the non-linear waveguides 121 and 122, respectively. A control light pulse con is inputted to the non-linear waveguides 121 and 122 at different timings from each other.
The control light pulse con is inputted to the non-linear waveguide 121 immediately after a pulse in a channel #1 has passed the non-linear waveguide 121, and is inputted to the non-linear waveguide 122 immediately after a pulse in a channel #2 has passed the non-linear waveguide 122. Therefore, the phase of an optical pulse in each of the channels #2 to #4 of the optical signal sig11 is shifted xcfx80 after passing the non-linear waveguide 121, and the phase of an optical pulse in each channel #3 and #4 of the optical signal sig12 is shifted xcfx80 after passing the non-linear waveguide 122.
When the optical signals sig11 and sig12 are combined with each other, the signals in the channels #1, #3 and #4 are introduced to one output optical fiber 125, and the signal in the channel #2 is introduced to the other output optical fiber 126.
Thus, in the Mach-Zehnder type optical demultiplexer, two arms, in which non-linear waveguides are respectively inserted, must be arranged parallel to each other. The device size is therefore increased.
FIG. 9C is a schematic view of a polarization separating type optical demultiplexer. An optical signal sig20 enters a birefringence crystal 130. The birefringence crystal 130 delays a light in the TM mode by one pulse relative to a light in the TE mode. An optical signal sig21 having passed the birefringence crystal 130 and a control light pulse con are both inputted to a non-linear waveguide 131. The control light pulse con is inputted to the non-linear waveguide 131 immediately after a TE-mode pulse in the channel #2 has passed the non-linear waveguide 131.
In an optical signal sig22 having passed the non-linear waveguide 131, therefore, the phase of the TE-mode optical pulse in each channel #3 and #4 is shifted xcfx80, and the phase of the TM-mode optical pulse in each of the channels #2 to #4 is shifted xcfx80. The optical signal sig22 having passed the non-linear waveguide 131 is inputted to another birefringence crystal 132. The birefringence crystal 132 delays a light in the TE mode by one pulse relative to a light in the TM mode. Accordingly, in an optical signal sig23 having passed the birefringence crystal 132, positions of the TM-mode pulses match respectively with positions of the TE-mode pulses in the corresponding channels.
In the optical signal sig23, therefore, the TM-mode pulses and the TE-mode pulses are in phase in the channels #1, #3 and #4, but they have a phase difference therebetween in the channel #2. By introducing the optical signal sig23 to enter a polarizer 133, only the pulse of the chancel #2 can be separated.
Thus, the polarization separating type optical demultiplexer is designed on condition that an inputted optical signal has intensities substantially equal to each other between the TM and TE modes. In general, however, the polarization state of an optical signal having propagated through an optical fiber is not constant. For that reason, the polarization separating type optical demultiplexer is not suitable for practical use.
While the method of demultiplexing an optical signal has been described above, those three types of optical demultiplexers can also be employed to operate as a wavelength converter by using a signal light and a control light having different wavelengths from each other.
As described above, the various types of conventional optical demultiplexers have problems such as a limitation in processing speed, an increased device size, and dependency on the polarization state of an optical signal.
It is an object of the present invention to provide an optical signal processing device and method, which can increase the processing speed, can reduce the device size, and are free from dependency on the polarization state of an optical signal.
Another object of the present invention is to provide an optical demultiplexer using the optical signal processing device.
Still another object of the present invention is to provide a wavelength converting device and method, which can increase the processing speed, can reduce the device size, and are free from dependency on the polarization state of an optical signal.
According to one aspect of the present invention, there is provided an optical signal processing device comprising an optical path superposing and separating unit for receiving two signal lights, superposing once optical paths of the two inputted signal lights with each other, and then separating the two signal lights to be outputted separately, the optical path superposing and separating unit including a non-linear waveguide arranged in an area where both the optical paths are superposed with each other, the non-linear waveguide having a refractive index changed depending on externally applied excitation; a first optical waveguide having a signal light input end and an output end connected to the optical path superposing and separating unit, the first optical waveguide introducing the signal light to the optical path superposing and separating unit; a second optical waveguide having a signal light input end and an output end connected to the optical path superposing and separating unit, the second optical waveguide introducing the signal light to the optical path superposing and separating unit, the second optical waveguide having an optical path length from the input end thereof to the optical path superposing and separating unit, which is longer than an optical path length of the first optical waveguide from the input end thereof to the optical path superposing and separating unit; a control-light introducing optical system for introducing a control light to the non-linear waveguide; an interference separator for receiving the two signal lights and distributing the inputted signal lights depending on a phase difference between the two signal lights; a third optical waveguide for connecting the optical path superposing and separating unit to the interference separator, and introducing one of the signal lights outputted from the optical path superposing and separating unit to the interference separator; and a fourth optical waveguide for connecting the optical path superposing and separating unit to the interference separator, and introducing the other signal light outputted from the optical path superposing and separating unit to the interference separator, the fourth optical waveguide having a shorter optical path length than the third optical waveguide, the optical path length of the fourth optical waveguide being set such that a delay time of the signal light propagating through the second optical waveguide relative to the signal light propagating through the first optical waveguide is canceled at time when the two signal lights reach the interference separator.
The signal light inputted to the optical path superposing and separating unit after passing the second optical waveguide is delayed from the signal light inputted to the optical path superposing and separating unit after passing the first optical waveguide. The phase of the signal light passing the non-linear waveguide after the time, at which the control light is introduced to the non-linear waveguide, is changed. The phase of that signal light is delayed, e.g., xcfx80. At the time when the two signal lights reach the interference separator, the delay between both the signal lights is canceled. In a certain period of time, therefore, the phase of one signal light differs from that of the other. The interference separator separates a portion of the signal lights in which they are out of phase.
According to another aspect of the present invention, there is provided a wavelength converting device comprising an optical path superposing and separating unit for receiving two continuous lights having a first wavelength, superposing once optical paths of the two inputted continuous lights with each other, and then separating the two signal lights to be outputted separately, the optical path superposing and separating unit including a non-linear waveguide arranged in an area where both the optical paths are superposed with each other, the non-linear waveguide having a refractive index non-linearly changed upon a control light pulse having a second wavelength being introduced; a first optical waveguide having a continuous light input end and an output end connected to the optical path superposing and separating unit, the first optical waveguide introducing the continuous light to the optical path superposing and separating unit; a second optical waveguide having a continuous light input end and an output end connected to the optical path superposing and separating unit, the second optical waveguide introducing the continuous light to the optical path superposing and separating unit; a control-light introducing optical system for introducing a control light pulse to the non-linear waveguide; an interference separator for receiving the two continuous lights and outputting the light having the first wavelength only during a period in which a phase difference between the inputted two continuous lights satisfies a certain condition; a third optical waveguide for connecting the optical path superposing and separating unit to the interference separator, and introducing one of the continuous lights outputted from the optical path superposing and separating unit to the interference separator; and a fourth optical waveguide for connecting the optical path superposing and separating unit to the interference separator, and introducing the other continuous light outputted from the optical path superposing and separating unit to the interference separator, the fourth optical waveguide having a shorter optical path length than the third optical waveguide.
The phase of the continuous light passing the non-linear waveguide after the time, at which the control light is introduced to the non-linear waveguide, is changed. The phase of that continuous light is delayed, e.g., xcfx80. At the time when the two continuous lights reach the interference separator, one continuous light is delayed from the other. In a certain period of time, therefore, the phase of one continuous light differs from that of the other. The interference separator separates a portion of the continuous lights in which they are out of phase. A separated optical signal is in sync with the control signal. In other words, the wavelength of the control signal is converted into that of the continuous light.
According to still another aspect of the present invention, there is provided an optical demultiplexer comprising a plurality of drop devices, each of the drop devices having a control light input port to which a control light is applied, a signal light input port to which a signal light is applied, and a drop signal output port; a signal waveguide for branching a time-division multiplexed signal light and applying a plurality of branched signal lights respectively to the signal light input ports of the drop devices; and a control waveguide for branching one control light and applying a plurality of branched control lights to reach the corresponding drop devices at delays gradually shifted in units of a certain time, each of the drop devices having the same construction as the optical signal processing device set forth above.
According to still another aspect of the present invention, there is provided an optical demultiplexer comprising a number N (N is two or larger integer) of drop devices, each of the drop devices having a control light input port to which a control light is applied, a signal light input port to which a signal light is applied, and a drop signal output port; a signal waveguide for applying a signal light, which is time-division multiplexed at multiplicity of N and has a number N of channels, to the signal light input port of each of the drop devices; and a control waveguide for branching one control light into a number N of control lights and applying an i-th (i is an integer not smaller than 1 but not larger than N) one of the branched control lights to the control light input port of an i-th drop device, the signal waveguide and the control waveguide delaying one of the control light and the signal light relative to the other such that the control light applied to the i-th drop device is in sync with an i-th channel of the signal light applied to the i-th drop device, each of the drop devices having the same construction as the optical signal processing device set forth above.
According to still another aspect of the present invention, there is provided an optical demultiplexer comprising a number N (N is two or larger integer) of drop devices arranged from a first stage to an N-th stage, each of the drop devices having a control light input port to which a control light is applied, a signal light input port to which a signal light is applied, a drop signal output port from which the signal light is delivered in sync with inputting of the control light, and a through signal output port from which the signal light is delivered at least during a period in which the signal light is not delivered from the drop signal output port; a first signal waveguide for applying a time-division multiplexed signal light to the signal light input port of the first-stage drop device; a second signal waveguide for connecting the through signal output port of each drop device to the signal light input port of the drop device in a next stage; and a control waveguide for branching one control light and applying a plurality of branched control lights to reach the corresponding drop devices at delays gradually shifted in units of a certain time toward a most downstream stage, each of the drop devices having the same construction as the optical signal processing device set forth above.
According to still another aspect of the present invention, there is provided an optical signal processing method comprising the steps of branching a time-division multiplexed optical signal having a plurality of channels into a first optical signal and a second optical signal; introducing the first optical signal and the second optical signal to a non-linear waveguide such that the second optical signal is delayed a time corresponding to one channel from the first optical signal; changing a refractive index of the non-linear waveguide at first time, thereby changing phase of the optical signal in each channel passing the non-linear waveguide after the first time; introducing the first optical signal and the second optical signal, which are both outputted from the non-linear waveguide, to an interference separator such that the first optical signal is delayed a time corresponding to one channel from the second optical signal; and separating the optical signal in the channel, in which the first optical signal and the second optical signal are out of phase, among the corresponding channels of the first optical signal and the second optical signal.
According to still another aspect of the present invention, there is provided a wavelength converting method comprising the steps of branching a continuous light having a first wavelength into a first continuous light and a second continuous light; introducing the first continuous light and the second continuous light to a non-linear waveguide; changing a refractive index of the non-linear waveguide at first time by introducing, to the non-linear waveguide, a control light pulse having a second wavelength different from the first wavelength, thereby changing phases of the first continuous light and the second continuous light both passing the non-linear waveguide after the first time; introducing the first continuous light and the second continuous light, which are both outputted from the non-linear waveguide, to an interference separator such that the first continuous light is delayed a first delay time from the second continuous light; and outputting an optical signal having the first wavelength only during a period in which the first continuous light and the second continuous light are out of phase.
With the features set forth above, only a signal light in a desired period of time can be extracted by superposing and then separating optical paths of two signal lights in a non-linear waveguide, and introducing the two signal lights to a separation interferometer, while one of the two signal lights is delayed from the other.